Spectroscopy and Dynamics of Ultracold Atom+Molecule Systems

Experiments with ultracold atoms have achieved an unprecedented level of control. In contrast, molecules remain far more challenging to prepare and stabilize at ultralow temperatures due to their rich internal structure, including rotational, vibrational, and hyperfine degrees of freedom. Consequently, in contrast to atom+atom systems, atom+molecule collisions exhibit nontrivial quantum dynamics involving dense spectra of states and the formation of long-lived collision complexes. At the same time, carefully designed experiments, for example, those combining molecular and atomic ions, offer new opportunities to probe molecular spectroscopy.

In the first part, I describe infrared absorption spectroscopy of CaOH⁺. Using high-level ab initio methods, we determine the position and intensity of the O–H vibrational transition. These calculations support the assignment of the absorption spectrum of a single CaOH⁺ ion co-trapped with a Ca⁺ ion and detected via recoil spectroscopy [1].

I then address the complexity arising in neutral atom+molecule systems. I investigate the properties of intermediate triatomic complexes formed during nonreactive collisions between ultracold alkali-metal molecules and atoms. For the KRb (X¹ Σ⁺ ) + Rb (²S) system [2], we identify an energetically accessible conical intersection between the ground and first excited electronic states, accompanied by enhanced spin-rotation coupling, which may contribute to the experimentally observed hyperfine-to-rotational energy transfer. In the NaLi (a³ Σ⁺ ) + Na (²S) system [3, 4], nonadditive three-body interactions significantly reshape the potential energy surface. The combined effects of spin-spin and spin-rotation interactions, together with strong anisotropy, significantly influence the collision dynamics.

[1] Wu, Z., Duka, T., Isaza-Monsalve, M., Kautzky, M., Švarc, V., Turci, A., Nardi, R., Gronowski, M., Tomza, M., Furey, B. J., and Schindler, P. (2026) Infrared absorption spectroscopy of a single polyatomic molecular ion. under review in Nature

[2] Liu, Y.-X., Zhu, L., Luke, J., Babin, M. C., Gronowski, M., Ladjimi, H., Tomza, M., Bohn, J. L., Tscherbul, T. V., and Ni, K.-K. (2025) Hyperfine-to-rotational energy transfer in ultracold atom–molecule collisions of Rb and KRb. Nature Chemistry, 17, 688–694

[3] Park, J. J., Son, H., Lu, Y.-K., Karman, T., Gronowski, M., Tomza, M., Jamison, A. O., and Ketterle, W. (2023) Spectrum of Feshbach Resonances in NaLi + Na Collisions. Physical Review X, 13, 031018

[4] Karman, T., Gronowski, M., Tomza, M., Park, J. J., Son, H., Lu, Y.-K., Jamison, A. O., and Ketterle, W. (2023) Ab initio calculation of the spectrum of Feshbach resonances in NaLi+Na collisions. Physical Review A, 108, 023309